1. Field of the Invention
The present invention relates to plasma systems capable of thermal spraying of powdered materials for coating on a workpiece.
2. History of the Prior Art
It is known to provide a plasma system in which powders of metal or other materials are delivered to a plasma gun for introduction into a plasma stream produced by a plasma gun. The plasma stream which is created by a flow of inert gas in the presence of an electrical power source and typically in the presence of a low pressure source is directed from the plasma gun onto a workpiece or other target where the powder is deposited to form a coating. The powder, which may be preheated before introduction into the plasma stream at the gun, melts as it is entrained into and carried by the plasma stream so that a relatively dense coating is formed on the workpiece.
An example of such a plasma system is provided by U.S. Pat. No. 4,328,257 of Muehlberger et al , which patent issued May 4, 1982 and is commonly assigned with the present application. In the plasma system described in the Muehlberger et al. patent a low pressure source in the form of vacuum pumps is coupled to an enclosure containing a plasma gun and a workpiece to direct the plasma stream from the plasma gun to the workpiece at supersonic speeds. A powder feed mechanism heats and delivers powder into the side of the plasma gun for introduction into the plasma stream.
Earlier examples of plasma and plasma related systems providing heated powder are described in U.S. Pat. No. 3,598,944 of Weimar et al. which issued Aug. 10, 1971 and U.S. Pat. No. 3,839,618 of Muehlberger which issued Oct. 1, 1974. The Weimar et al. patent describes heating particulate matter before introduction into a plasma heating zone in a device for creating spherical granules of nuclear fuel. The Muehlberger patent describes a plasma system in which powder is preheated using an electrical resistance tube. Powder is fed from separate sources through a pair of such tubes using a heated carrier gas. The tubes heat the powder which is then introduced into the bore of the plasma gun. The tubes are resistively heated using a DC power supply coupled to the tubes.
In U.S. Pat. No. 3,839,618 of Muehlberger, powder particles are said to have a diameter which is no greater than 44 microns and preferably much less. The relatively small particle size of less than 50 microns typically required by heated powder delivery apparatus of the prior art is a characteristic limitation of such systems. The small particle sizes are required because of the ease with which such small particles can be preheated and then melted as they are rapidly accelerated in the plasma stream so as to produce a reasonably dense coating thereof on the workpiece.
However, small particles having diameters of less than 50 microns are not without their limitations. To begin with such particles are relatively expensive to manufacture, particularly in the case of materials such as refractory materials and getter materials. Refractory materials such as tungsten and molybdenum have relatively high melting points, while getter materials such as barium, titanium and tantalum oxidize rapidly. Moreover, such particles are of limited viscosity or flowability, making it relatively difficult to transfer them through apparatus such as heated tubes of relatively small inner diameter. Such small particles also tend to experience high surface oxidation, making it difficult to deliver the particles to the workpiece in a relatively pure, oxide-free form. The surface area to weight ratio of such particles tends to be relatively low, and the resulting poor thermal energy transfer makes it more difficult to control the heating of such particles.
On the other hand relatively large powder particles having diameters substantially in excess of 50 microns are advantageous over smaller particles in a number of important respects. In addition to being relatively inexpensive to manufacture, such particles have improved flowability. The ease of producing such particles in a highly pure form and the low surface oxidation typically undergone by such particles facilitate the production of a relatively dense, oxide-free coating on the workpiece. The major disadvantage of such large particles lies in the difficulty of achieving complete melting thereof within the plasma stream using conventional apparatus and techniques. The difficulty of melting such particles makes it very difficult to form a satisfactory coating on the workpiece. Even where such powders are preheated such as through the use of resistively heated delivery tubes, coating of the workpiece is difficult to accomplish.
The difficulty in thermal spraying relatively large particles heretofore relates to a more fundamental problem which has been a lack of understanding of the thermal spraying process in general. Heating of powder particles was often done in a standardized manner without regard to the different softening and melting characteristics of different materials as well as the varying characteristics of different particle sizes noted above. Also, it has usually been assumed that control of powder temperature as well as other characteristics of the powder delivery must occur at or just outside of the plasma gun.
Accordingly, it would be desirable to have a plasma system in which thermal spraying can be controlled by varying all parts of the plasma system which affect such spraying in an optimum manner. Such an improved system approach would include the ability to successfully accomplish thermal spraying of a variety of different powder materials having different softening and melting characteristics as well as different particle sizes.
More specific goals of an improved plasma system would include the ability to spray powdered materials of relatively large particle size in a manner producing a relatively dense and uniform coating on the workpiece. It would also be advantageous to provide heated powder delivery apparatus capable of preheating powder particles of any size so that the particles undergo substantially complete melting within the plasma stream, while at the same time providing a relatively smooth and continuous flow of the particles through the powder delivery apparatus.